Well, those two classes are both effective in their own way (as most classes are in their own specific way)
1. If you go as Para, HM would be tad easy since you could run as imbagon and it'll be a breeze for titles. As for Sin, your wide variety of skills and your Crit mastery is very useful in any/most build.
2. Party wise, Paragons are more wanted due to their damage mitigation. Sins, on the other hand, have fast DPS so they could kill easier and faster (combine it with an SoH + GDW + JI Monk an you'll surely see the 100s flow).
3. Farming? Assassins have a variety to choose from; Paragons have one or even none since in my opinion they were really created for team play.
4.1. Well basically I enjoy most being a Para (and it's my main btw) since they support backline and they themselves are strong because:
a. when you have the 2 Para heroes already you'll see that your effects bar/enchantments bar or w/e you call it is full of shouts and whatnots that a Paragon has in advantage.
b. they have good white damage (with a 16 in spear and a 20/20 they do around 40-60 on 60AL) then combine it with either a good elite or some pve skills then you're bound to be a ranged warrior.
4.2. As for Assassins you may want to be this more because of its continuous combos and good damages plus the advantage of most groups for speed clears.
To summarize it all up:
ASSASSIN | PARAGON
FARMING Yes | No
PVE FRIENDLY Yes | Yes
EFFICIENCY Yes | Yes
Hope that helps you in deciding whether a Para or Sin